Title

Exchange nonexistent recipients must not be blocked. (Cat II impact)

Discussion

Spam originators, in an effort to refine mailing lists, sometimes use a technique where they first create fictitious names and then monitor rejected emails for non-existent recipients. Those not rejected are deemed to exist and are used in future spam mailings. To prevent this disclosure of existing email accounts to spammers, email to nonexistent recipients must not be blocked. Instead, it is recommended that all messages be received, then evaluated and disposed of without enabling the sender to determine existent vs. nonexistent recipients.

Check Content

Note: If third-party anti-spam product is being used, the anti-spam product must be configured to meet the requirement. Open the Exchange Management Shell and enter the following command: Get-RecipientFilterConfig | Select Name, RecipientValidationEnabled If the value of "RecipientValidationEnabled" is not set to "False", this is a finding.

Fix Text

Open the Exchange Management Shell and enter the following command: Set-RecipientFilterConfig -RecipientValidationEnabled $false

Additional Identifiers

Rule ID: SV-221236r879653_rule

Vulnerability ID: V-221236

Group Title: SRG-APP-000261

Expert Comments

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